The present invention concerns a way of printing, by means of tampon printing of a conductive coating on a detail, such as the casing of an electrical or electronic unit, such as a telephone.
When an electrical or electronic unit, such as a telephone is to be shielded against electromagnetic radiation, such as ESD currents, the unit is surrounded at least partly by a conductive material. The conductive material consists of a conductive layer or coating e.g. printed by means of tampon printing on the surface of the detail.
At tampon printing a picture is created on a detail by a method which starts in that paint is raked out by a so called cup, filled with paint, against a clichxc3xa9. This is embossed with a certain depth and the paint will be trapped in the embossing and thus a picture is formed, which then will be fetched by the tampon pad. This is then moved and goes down towards the detail and is pressed against it. The tampon pad is soft and the picture will adhere to the detail.
Normally smooth surfaces are printed without any mayor problems when using tampon printing of the prior art. At corners, ribs, orifices, screw sockets, snaps, protruding parts, walls etc. it is hard to reach the surface and print the conductive coating when using a tampon pad. However it is known to rotate the detail or to alter the angle of attack of the tampon pad, in order to have a new printing direction. Hereby the tampon pad will more easily reach the surface and give a better print on the surface of the detail. This is often not enough to get a satisfactory result as the most difficult parts of the surface to reach will not be printed and there will be holes in the conductive coating. Furthermore, the coating will be too thin at some parts. If the above problems occur electromagnetic radiation will leak and the resistance of the coating increases and thereby will the conductivity decrease. Costly and time-consuming subsequent treatment will be needed, in form of an extra inlay of a conductive layer by means of spray painting using precision nozzles, tampon printing with extremely small tampon pads or many extra tampon prints from various angles of attack. These techniques are costly and defective.
When using other methods than tampon printing, such as spraying and vacuum evaporation of conductive coatings, insufficient results will often arise. This is due to the fact that grooves, corners, snaps, screw sockets etc. will create a shadowing effect leading to that parts of the surface to be spray painted or vacuum evaporated will not be coated. Thus, a completion of the surface must be made, in order to improve the shadowed parts, which also will be costly and time-consuming.
One object of the present invention is to eliminate the drawbacks of the above methods by letting the tampon pad move in small movements in one or more directions in relation to an electronic unit, at the same time as the tampon pad is in contact with the surface to be printed.
Other objects and advantages of the present invention will be obvious for a person skilled in the art when reading the detailed description below.
The method of the invention is especially suitable for surfaces that are hard to reach such as at the corners of the casing, at screw sockets, at ribs or the like. This is due to the fact that the tampon pad is moved in different directions during the phase of tampon printing, and thus the pad will rub against the surface to be printed. Hereby the conductive layer will be better worked into the surface of the detail. Of course this method will get a better result for smooth surfaces than the present way of tampon printing.